1. Field of the Invention
This invention relates to ferritic stainless steel sheets having excellent deep-drawability and surface smoothness applicable to home electric appliances, kitchen appliances, construction, and automobile components and to methods for making the same. In particular, the invention relates to a ferritic stainless steel sheet suitable for use in automobile fuel tanks and fuel pipes which are made by high deformation such as deep drawing and pipe expanding, and are highly resistant to organic fuels such as gasoline and methanol which contain organic acids produced in the ambient environment. A method for making the same is also provided.
2. Description of the Related Art
Ferritic stainless steels which do not contain large amounts of nickel (Ni) are cost effective compared with austenitic stainless steels and are free of stress corrosion cracking (SCC). Due to these advantages, ferritic stainless steels have been used in various industrial fields. However, known ferritic stainless steels exhibit low elongation of approximately 30% and are thereby inferior to austenitic stainless steels, for example, SUS 304, in workability. Known ferritic stainless steels do not have sufficient workability for high deformation such as deep drawing, and typically, press forming, and are not suitable for mass production. Because of these problems concerning formability, the use of ferritic stainless steel in various fields such as automobiles, construction, and home electric appliances has been severely limited.
Several attempts have been made to improve the formability of ferritic stainless steels. Among these, Japanese Unexamined Patent Publication No. 3-264652 proposes optimization of manufacturing conditions of ferritic stainless steels containing Nb and Ti in order to obtain an aggregation structure of 5 or more in X-ray intensity ratio (222)/(200) and to improve the formability.
In this technology, however, the r-value is only about 1.8; hence, application to fuel tanks requiring complex forming by deep drawing and to fuel pipes requiring pipe-expansion and bending is difficult. Moreover, even if applied at all, defect rates are high and mass production is not practical. On the other hand, ternesheets, i.e. soft steel sheets provided with plating containing lead, have been widely used as the material for automobile fuel tanks. However, regulations on the use of lead are becoming stricter from an environmental point of view and substitutes for the ternesheets have been developed. The substitutes developed have the following problems. Lead-free Alxe2x80x94Si based plating materials are unreliable in terms of weldability and long-term corrosion resistance and the application thereof is thus limited. Resinous materials have been applied to fuel tanks, but since these materials naturally allow minute amounts of fuel to permeate, the industrial use thereof is inevitably limited under fuel transpiration and recycling regulations. Use of austenitic stainless steels which can be used without lining have also been attempted. Although austenitic stainless steels are superior in formability and corrosion resistance to ferritic stainless steels, they are expensive for use in fuel tanks and may suffer from stress corrosion cracking (SCC). Thus, the use of austenitic stainless steels has not been practical.
In such a situation, enormous advantages such as improvement of the global environment can be achieved if these materials can be substituted by ferritic stainless steels which are recyclable.
Since the r-value of ternesheets is approximately 2.0, ferritic stainless steels must attain an r-value of 2.0 or more for them to replace the ternesheets. Ferritic stainless steels must also have long-term corrosion resistance to deteriorated gasoline containing organic acids such as formic acid and acetic acid which are formed in the ambient environment in order for the ferritic stainless steels to be applied to fuel components such as automobile fuels tanks and pipes. However, no investigation has specified suitable compositions for attaining these goals.
As previously described, the r-value of the known ferritic stainless steels is only approximately 2.0 at most, and application of ferritic stainless steels to pressed components requiring extensive deep drawing has not been achieved. Another problem with ferritic stainless steels is the generation of rough surfaces after pressing by deep drawing. Here, rough surfaces include the orange peel condition caused by rough crystal grains and the presence of corrugations aligned in the rolling direction (L direction) as a result of cold rolling thereby rendering undulating surfaces in the sheet width direction.
In view of the above, a first object of the invention is to provide a ferritic stainless steel exhibiting enhanced deep-drawability which is suitable for application to automobile fuel tanks and pipes by improving the r-value to 2.0 or more and provide a method for making the same.
In particular, an object of the invention is to provide a ferritic stainless steel exhibiting an average r-value as the parameter of deep-drawability of 2.0 or more, preferably about 2.2 more, having a crystal grain size number in the finished annealed sheet as the parameter of the surface-roughness of about 6.0 or more, and developing no red rust after corrosion resistance testing using deteriorated gasoline containing 800 ppm of formic acid at 50xc2x0 C. for 5,000 hours.
The average r-value is defined as the average plastic strain ratio according to Japanese Industrial Standard (JIS) Z 2254 calculated using the equation below:
r=(r0+2r45+r90)/4
wherein,
r0 denotes a plastic strain ratio measured using a test piece sampled in parallel to the rolling direction of the sheet;
r45 denotes a plastic strain ratio measured using a test piece sampled at 45xc2x0 to the rolling direction of the sheet; and
r90 denotes a plastic strain ratio measured using a test piece which is sampled at 90xc2x0 to the rolling direction of the sheet.
Another object of the invention is to solve the problems conventionally experienced during forming the ferritic stainless steel sheets into fuel tanks and pipes of severe shapes and during a process such as pressing which requires omission of application of vinyl lubricant or oil.
Based on our research, we found that application of a lubricant coat containing acrylic resin as the primary component on the surface of the steel sheet at an amount within a predetermined range improves the sliding property during press forming and reduce the dynamic friction coefficient between the ferrite stainless steel and pressing dies. Thus, xe2x80x9cgallingxe2x80x9d can be prevented and products of further complicated shapes can be manufactured.
In order to attain the above-described objects, we conducted extensive research on improvement of the corrosion resistance with deteriorated gasoline, deep drawability, and surface roughness after processing required for applying ferritic stainless steels to automobile fuel components. We found that the corrosion resistance with deteriorated gasoline can be effectively improved by including about 0.5 mass percent (hereinafter, simply referred to as %) of Mo, controlling the sum Cr+3.3Mo (pitting index) to not less than about 18%, and inhibiting the rough surface after processing. We also found that the disadvantages of including large amounts of Mo, i.e., degradation in deep drawability and generation of rough surfaces, can be overcome by performing cold rolling at least twice with an intermediate annealing process therebetween and by optimizing the manufacturing conditions such as crystal grain sizes during cold rolling. Moreover, we found that the dynamic friction coefficient between ferritic stainless steel sheets and dies can be reduced by coating the steel sheet surface with a lubricant coat to improve sliding properties during forming. Thus, the ferritic stainless steel sheets can be formed into products having more complex shapes.
To achieve these objects, an aspect of the invention provides a ferritic stainless steel sheet having an average r-value of at least 2.0 and a ferrite crystal grain size number determined according to Japanese Industrial Standard (JIS) G 0552 of at least about 6.0, the ferritic stainless steel sheet comprising, by mass percent:
not more than about 0.1% C, not more than about 1.0% Si, not more than about 1.5% Mn, not more than about 0.06% P, not more than about 0.03% S, about 11% to about 23% Cr, not more than about 2.0% Ni, about 0.5% to about 3.0% Mo, not more than about 1.0% Al, not more than about 0.04% N, at least one of not more than about 0.8% Nb and not more than about 1.0% Ti, and the balance being Fe and unavoidable impurities, satisfying relationship (1):
18xe2x89xa6Nb/(C+N)+2Ti/(C+N)xe2x89xa660xe2x80x83xe2x80x83(1)
wherein C, N, Nb, and Ti in relationship (1) represent the C, N, Nb, and Ti contents by mass percent, respectively.
The Cr and Mo contents may satisfy the relationship (2):
Cr+3.3Mo xe2x89xa718xe2x80x83xe2x80x83(2)
wherein Cr and Mo represent in relationship (2) represents the Cr and Mo contents by mass percent, respectively.
Preferably, the X-ray integral intensity ratio (222)/(200) at a plane parallel to the sheet surface is not less than about 15.0.
Preferably, the ferritic stainless steel sheet is bake-coated with a lubricant coat comprising an acrylic resin, calcium stearate, and polyethylene wax in a coating amount of about 0.5 to about 4.0 g/m2.
Another aspect of the invention provides a method for making a ferritic stainless steel sheet, the method comprising the steps of:
preparing a steel slab containing not more than about 0.1% C, not more than about 1.0% Si, not more than about 1.5% Mn, not more than about 0.06% P, not more than about 0.03% S, about 11% to about 23% Cr, not more than about 2.0% Ni, about 0.5% to about 3.0% Mo, not more than about 1.0% Al, not more than about 0.04% N, at least one of not more than about 0.8% Nb and not more than about 1.0% Ti, and the balance being iron (Fe) and unavoidable impurities, satisfying relationship (1):
18xe2x89xa6Nb/(C+N)+2Ti/(C+N)xe2x89xa660xe2x80x83xe2x80x83(1)
where C, N, Nb, and Ti in relationship (1) represent the C, N, Nb, and Ti contents by mass percent, respectively;
heating the steel slab at a temperature in the range of about 1,000xc2x0 C. to about 1,200xc2x0 C., hot-rough-rolling the steel slab at a rolling temperature of at least one pass of about 850xc2x0 C. to about 1,100xc2x0 C. by a reduction of about 35%/pass, hot-finish-rolling the slab at a rolling temperature of at least one pass of about 650xc2x0 C. to about 900xc2x0 C. by a reduction of about 20 to about 40%/pass to prepare a hot-rolled sheet;
annealing the hot-rolled sheet at a temperature in the range of about 800xc2x0 C. to about 1,100xc2x0 C.;
cold-rolling the resulting annealed sheet at least twice with intermediate annealing therebetween, said cold rolling being performed at a gross reduction of about 75% or more and a reduction ratio (reduction in the first cold rolling)/(reduction in the final cold rolling) in the range of about 0.7 to about 1.3; and
finish annealing the cold-rolled sheet at a temperature in the range of about 850xc2x0 C. to about 1,050xc2x0 C.
Preferably, the Cr and Mo contents in the steel slab satisfy the relationship (2):
Cr+3.3Moxe2x89xa718xe2x80x83xe2x80x83(2)
wherein Cr and Mo in relationship (2) represent Cr and Mo contents by mass percent, respectively.
Preferably, the grain size number of ferrite crystal grains of the steel sheet before the final cold rolling measured according to JIS G 0552 is not less than about 6.5.
Preferably, said step of cold rolling is performed in a single direction using a tandem rolling mill comprising a work roller having a diameter of about 300 mm or more.
The method for making the ferritic stainless steel sheet may further comprise the step of bake-coating the finish-annealed ferritic stainless steel sheet with a lubricant coat comprising an acrylic resin, calcium stearate, and polyethylene wax in a coating amount of about 0.5 to about 4.0 g/m2.